Fix error when calling close() on a dgram socket multiple times

[heimskr]

What does this PR do?

Prevents a property lookup on null when closing the same dgram socket multiple times.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

Tried closing a socket multiple times.

[robobun]

^{Updated 5:58 PM PT - Jan 30th, 2025}

❌ @heimskr, your commit cbdbcbe has 2 failures in Build #10887:

• test/js/bun/util/filesink.test.ts - timeout on 🪟 2019 x64-baseline
• test/js/bun/util/filesink.test.ts - timeout on 🪟 2019 x64
• test/js/node/test/parallel/test-fs-promises-writefile.js - code 1 on 🪟 2019 x64-baseline

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🧪   try this PR locally:

bunx bun-pr 16928